Pretty simple question today: can I hold priority during my opponent's turn and respond to my own spell or ability?
Yes.
116.3c If a player has priority when he or she casts a spell, activates an ability, or takes a special action, that player receives priority afterward.
The tournament rules state that you need to be explicit about it. (And I understand that in MTGO, you need to hold down the control key at a certain point.)
Certain conventional tournament shortcuts used in Magic are detailed below. They define a default communication; if a player wishes to deviate from these, he or she should be explicit about doing so. Note that some of these are exceptions to the policy above in that they do cause non-explicit priority passes.
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Whenever a player adds an object to the stack, he or she is assumed to be passing priority unless he or she explicitly announces that he or she intends to retain it.
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Why bother with mere rulings when so many answers can be found in the Rules?
The tournament rules state that you need to be explicit about it. (And I understand that in MTGO, you need to hold down the control key at a certain point.)